Improving the Catalytic Activity of Metal Complexes and Pyrolyzed Non-Precious Metal- N4 Catalysts for the Reduction of O2

Abstract

The massive applications of fuel-cells in daily life has been hampered in part by the high cost of catalytic materials for the O2 cathode that involve noble metals like Pt. MN4 metal macrocyclics [1] and several CuN4 and CuN2 complexes [2] are active for the reduction of O2 but they lack long-term stability in fuel conditions. However, they could useful in disposable air-batteries. Many authors have shown (3-4) that heating MN4 metal macrocyclics together with carbonaceous materials and conducting polymers at temp -eratures as high as 1000 oC produce materials of ill-defined structure but more active and stable. The MNx inner structure is probably retained. It has remained unclear why the heat-treated materials exhibit such high catalytic activity. In many papers (1) we have demonstrated that the catalytic activity of intact MN4 metal complexes is directly linked to the M(III)/(II) formal potential, the more positive the highest the activity and this is also valid for Cu N4 and CuN2 complexes [2] For example for Fe-and Co-containing MN4 catalysts a plot of (log i)E versus the formal potential Eo M(III)/(II) of the catalyst gives a linear correlation with a slope of +0.160 V/decade, i.e. a value that is close to a Tafel slope of -0.120 V/decade but with opposite sign. The redox potential of the catalyst can be tuned by preparing MN4 chelates with appropriate groups located on the ligand or by axial ligation. [1] We have proposed that the very high activities of the heat-treated CNxM materials (x=2,4) can be explained by the shift of the formal potential of the MN4 moiety to more positive values [5] and this has been supported by recent findings of Mukerjee et al. that correlate the activity with the Lewis basicity of the graphitic support, accessed via C 1s photoemission spectroscopy [6]. All this suggests that preparing heat-treated materials that would have redox couples more shifted to more extreme positive potentials (ca 0.8 V or more) by creating an electron-acceptor environment around the metal that could survive the high pyrolyzing temperatures. Many catalysts heat-treated at 1000oC do not exhibit clear redox couples but this is the right direction to go and hypothetically it is possible to overpass the catalytic activity of Pt. A volcano correlation has not yet been found for intact and pyrolyzed MN4 systems so to prepare catalysts that have redox potentials beyond 0.8 V or closer to the thermodynamic potential of the H2O/O2couple (1.22 V vs NHE) should provide the highest possible activity. In this paper we show several correlations using many complexes and pyrolyzed MN4 catalysts which show that there is a clear hope of obtaining catalysts better than Pt and having cheaper fuel-cells for broad applications. Some other explanations of these trends will be discussed that propose a new semi-empirical model of electrocatalysis. Acknowledgements: This work has been funded by Fondecyt 1100773, Dicyt-USACH and Núcleo Milenio Project. P07-006